Chances are your magnetron is overheating, activating the magnetron thermostat, which resets and restores power when it cools.
Your manual may call for you to put a paper plate under the popcorn bag to heat better and to protect the turntable. (And always place the food slightly off-center on the turntable, for the same reasons!)
You should first confirm that the outlet reads about 110-120 volts AC and that the microwave is on its own breaker. If a microwave shares a breaker with another high current device (or even lights) it can affect operation.
Then be sure the microwave's internal cooling fan is running.
If this is an over the range model, it will also have a exhaust hood vent fan which you can turn on and off at will, but this is not the fan I'm talking about.
The internal cooling fan should be on whenever the oven is cooking. If not, the magnetron can overheat and eventually be destroyed.
The next issue is how long you've had the oven.
If it's within the 30-day dealer exchange period, I'd take it back for a new one pronto.
If it's within the full warrranty period (usually 1 year) Daewoo's service center should fix it free.
You can visit Daewoo online
or call them at 1-800-323-9668 or 1-800-553-4355 to find parts or find
your nearest authorized service center to have them do the work.We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.